1. Field of the Invention
The present invention relates to control circuits for light-emitting diodes (LEDs), and particularly to a related integrated circuit and method for controlling LEDs.
2. Description of the Prior Art
Light-emitting diodes (LEDs) have a very good electro-optical conversion rate, which is higher than fluorescent lamps, cold-cathode fluorescent lamps, and light bulbs. Thus, the current trend is to replace these types of lamps with LEDs. For example, LEDs have already gradually replaced CCFLs as a backlight source in liquid crystal display (LCD) panels.
FIG. 1 is a diagram of an LED voltage supply circuit. Grid AC power goes through power management circuit 12, is transformed by transformer 14, and outputted through terminal OUT as output voltage VOUT. Through voltage divider 18, 20, Zener diode LT431, and photocoupler 16, output voltage VOUT is fed back to power management circuit 12, so as to regulate output voltage VOUT. N LED chains CLED1 . . . CLEDN are connected to output voltage VOUT as light sources. Controllable current sources I1 . . . IN have approximately the same internal circuitry, each corresponds individually to one of the LED chains CLED1 . . . CLEDN, and controls LED chain brightness by controlling current flowing through each LED chain.
FIG. 2 illustrates one configuration of controllable current source I1 of FIG. 1. Error amplifier EA1 controls power transistor M1, and changes current flowing through LED chain CLED1 and sense resistor CS1, in an attempt to make voltages of sensing signal VSENSE1 and current-setting signal VLEDBIAS the same. Sense resistors CS1 . . . CSN have approximately the same resistance. If sensing signals VVSENSE1 . . . VSENSEN have voltage approximately equal to current-setting signal VLEDBIAS, it can be deduced that current flowing through each LED in FIG. 2 is approximately the same.
Short and open circuits among the circuit components, including LED current sources, lead to abnormal operation, and can even cause mortal danger. For example, if the sense resistor CS1 of FIG. 2 is shorted (resistance is zero), brightness of LED chain CLED1 will be overly bright, or LED chain CLED1 may even burn up. Thus, errors should be detected as early as possible, so as to implement appropriate protection measures.